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Post subject: Difference between Class A,B and Class AB amp operation
Posted: Sat Jul 12, 2014 9:05 am
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Aspiring Musician
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For those of you who may be electrical or electronics engineers here,...my question is this. What is the difference in operation in these Classes of amps and their operation? What amps would fall under which class? I know most,if not all,modern Fender amps fall under Class AB operation. One of my radio buddies and I have had an arguement concerning tube amps and he says a LOT of the stuff you read on sites such as this one is BS and that you DON'T have to have a matched set of power tubes and you can use ANY 6L6 tube you want to and won't have cross distortion. (He says that since I am just a beginner I shouldn't care about tone,...well,I DO give a damn about my tone whether I am pro or amateur/beginner. He doesn't even PLAY guitar.) Okay,...to try and clarify this a bit,...what amps require a matched set of power tubes and which would not? That is the question I need answered! He also tries to make claim that Fender only tells us we should use a matched set of tubes just to cover their butts in case of warranty issues and in reality matched set of tubes are,...again,.. NOT necessary and it's BS if the manufacturer and amp techs tell you that. Now,...would amps such as a Vox AC15 and AC30 fall under Class a operation? What about others?

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If it aint' broke,...don't fix it! I like to keep my amps and guitars factory original,...no mods necessary,...don't want them,...don't need them!

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Post subject: Re: Difference between Class A,B and Class AB amp operation
Posted: Sat Jul 12, 2014 10:05 am
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A "cycle" is one complete positive and negative swing of the wave form (sine wave). A sine wave is the AC signal produced from a guitar that is amplified by the guitar amplifier. This signal alternates from positive to negative at various frequencies while playing.

Class A simply means that a tube is always conducting the signal during the cycle, it doesn't stop conducting at the low swings in the cycle. You typically only see class A amps as a single-ended configuration, almost always with only one output tube. Think Champ, Vibro Champ, etc. There are amps such as many Vox, that operate in Class A using "push-pull"pairs of output tubes instead of single ended. There are also amps that use a single-ended configuration with more than one output tube, not in a "push-pull" configuration.

Class B means that a tube only conducts for half of the cycle, it stops conducting on the low swings in the cycle. Not many instrument amps use Class B operation. Maybe Ham radios use Class B, I don't know.

Class AB uses pairs of output tubes, say two or four, and each half of the signal, positive or negative is controlled by one half of the pair of tubes, in what is referred to as a "push-pull" pair. One half of the tube pair conducts during the negative half of the signal (Class B) and when the signal swings back to zero, that half stops conducting at about the time that the other half of the pair takes over and conducts for the positive half of the signal (class B). When the two halves of the signal are combined by the OT, you get what is called class AB operation. Meaning that although each half of the output pair is operating in Class B, the combining of the two halves by the OT looks like Class A, because the signal is never "off", hence Class AB.

The importance of matched output tubes in class AB operation is to balance the work done between the two halves of the signal. If one half of the signal is being amplified more than the other half, the sound might not be optimal, although there are those that like an imbalanced output, harp players come to mind. Think of a Harley V-Twin in which one of the cylinders is doing more work than the other and that might give you an idea of how an imbalanced output in an instrument amplifier would function.

Your buddy sounds like a typical radio guy with no understanding of instrument amps at all. Tell him to go sit on his antenna. :lol:

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Last edited by shimmilou on Sat Jul 12, 2014 10:18 am, edited 1 time in total.

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Post subject: Re: Difference between Class A,B and Class AB amp operation
Posted: Sat Jul 12, 2014 10:14 am
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shimmilou wrote:
Your buddy sounds like a typical radio guy with no understanding of instrument amps at all. Tell him to go sit on his antenna.


AND its base-mounted loading coil.

:mrgreen:

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Difference between Class A,B and Class AB amp operation
Posted: Sat Jul 12, 2014 10:43 am
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Aspiring Musician
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Retroverbial wrote:
shimmilou wrote:
Your buddy sounds like a typical radio guy with no understanding of instrument amps at all. Tell him to go sit on his antenna.


AND its base-mounted loading coil.

:mrgreen:

Arjay


:lol: :lol: :lol: :lol: :lol: :lol: :lol: :lol: Good one! Anyway,thanks for the response and clarifying things. I am now getting a far better understanding of how a tube amp functions
and learning more and more each day just researching this site and others like it. Man I wish I had the knowledge you guys have,...if I did,...I'd be an amp tech,...not a forklift operator in a grocery store warehouse. :)

_________________
If it aint' broke,...don't fix it! I like to keep my amps and guitars factory original,...no mods necessary,...don't want them,...don't need them!

My gear as of now;
Standard Strat
Hot Rod Deluxe III amp
Champion 20 amp
'59 Bassman LTD Reissue


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Profile
Post subject: Re: Difference between Class A,B and Class AB amp operation
Posted: Sat Jul 12, 2014 12:37 pm
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KC9SYJ wrote:
if I did,...I'd be an amp tech,...not a forklift operator in a grocery store warehouse. :)


People always gotta eat -- thus, you'll always have a job.

Not so with the esoteric/ephemeral nature of musical instruments, and their ancillary support equipment.

Consider yourself lucky.

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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Post subject: Re: Difference between Class A,B and Class AB amp operation
Posted: Sat Jul 12, 2014 2:52 pm
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Aspiring Musician
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Joined: Sun Jan 27, 2013 3:44 pm
Posts: 366
Retroverbial wrote:
KC9SYJ wrote:
if I did,...I'd be an amp tech,...not a forklift operator in a grocery store warehouse. :)


People always gotta eat -- thus, you'll always have a job.

Not so with the esoteric/ephemeral nature of musical instruments, and their ancillary support equipment.

Consider yourself lucky.

Arjay



True,...you have a valid point there!

Dewey
:mrgreen:

_________________
If it aint' broke,...don't fix it! I like to keep my amps and guitars factory original,...no mods necessary,...don't want them,...don't need them!

My gear as of now;
Standard Strat
Hot Rod Deluxe III amp
Champion 20 amp
'59 Bassman LTD Reissue


Top
Profile
Post subject: Re: Difference between Class A,B and Class AB amp operation
Posted: Sat Jul 12, 2014 7:25 pm
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Decades ago I did some consulting work for an aerospace/defense contractor and later became friends with the CEO due to our shared interest in an esoteric hobby. I told him he was making the wrong type of product since there was limited demand and few prospects for adapting the company to emerging technologies. When asked what I thought he should be making I told him jokingly, "olive-green paint". That way no matter who the customer was or what type of equipment needed camouflage, there would always be a market. A dozen years later, the company abandoned its original product line and began developing and selling a proprietary line of protective coatings for worldwide military, maritime, and aerospace applications. My former acquaintance started his business in 1972 with a paltry $650K loan from the SBA and sold it in 2003 for some $330M.

Rain or shine, recession or recovery, and war or peace, there will always be a market for olive-green paint.

And grocery stores.

:wink:

Arjay

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"Here's why reliability is job one: A great sounding amp that breaks down goes from being a favorite piece of gear to a useless piece of crap in less time than it takes to read this sentence." -- BRUCE ZINKY


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